Continuous process for production of cellulose pulp from grass-like feedstock

Abstract

An improved continuous process for production of cellulose from glass-like feedstock, which includes:(i) cooking of comminuted grass-like feedstock in white liquor which contains 0.5-2.0% w/w NaOH and 0.5-25.0% w/w NaCl at 95-100° C.;(ii) bleaching process of the brown cellulose in the white liquor of the same composition with oxygen (O2) and chlorine (Cl2) at 70-100° C., yielding bleached cellulose pulp; where,(iii) lignin and other side-products are separated by continuous electrolysis, where the white liquor is regenerated, together with O2 and Cl2 for the bleaching purpose, thereby closing the cycle of the process.

The invention claimed is: 
     
       1. A continuous process, for production of cellulose pulp from a grass-like feedstock, that comprises an electrolytic process for continuous electrolytic separation of lignin and non-cellulosic side-products, with simultaneous production of white liquor and oxygen <O 2 > and chlorine <Cl 2 >, which is carried out in an electrolytic cell system which includes:
 one or more electrolytic pre-cells, made from a first material inert to chemistry in which first cathode and anode electrodes are immersed, with no membrane between respective first cathode and anode compartments; 
 one or more electrolytic cells, made from a second material inert to chemistry in which second cathode and anode electrodes are immersed, where a second cathode compartment is completely separated from a second anode compartment with a porous membrane, which enables electrical contact of an anode and a cathode respectively associated with the second anode and cathode electrodes via ion-exchange, but prevents passing of suspended organic molecules in electrolyte; 
 where the electrolyte is, after processing in the one or more electrolytic pre-cells, transferred into the second anode compartment of the one or more electrolytic cells, and a composition of the electrolyte is modified by addition of a fresh sodium chloride (NaCl) solution into the one or more electrolytic pre-cells; 
 the continuous process comprises the following steps: 
 A) preparing a suspension of comminuted and dedusted grass-like feedstock by addition of a white liquor from the second cathode compartment of the one or more electrolytic cells into a preliminary digester where the grass-like feedstock is subjected to cooking at 80-100° C. in a solution of the white liquor including the following composition: 
 (ii) 0.50-2.00 w/w NaOH; 
 (ii) NaCl; 
 by keeping a concentration of the grass-like feedstock at a level of 5-15% w/w; 
 B) performing quasi-continuous cooking and separation through two or more parallel cooking and separation processing lines each including one or more primary digesters, one or more cooking mills, one or more cooking water separators, and one or more secondary digesters, during 3-6 hours, wherein the grass-like feedstock prepared in step A is sequentially pumped into the two or more parallel cooking and separation processing lines, wherein: 
 suspension of the grass-like feedstock in the white liquor during cooking in all of the one or more primary digesters and the one or more secondary digesters is maintained at temperature from 95-100° C.; 
 for each of the two or more parallel cooking and separation processing lines, a brown cellulose suspension at a level 8-12% w/w dry matter at an output from the one or more primary digesters is subjected to milling in the corresponding one or more cooking mills and separation in the one or more cooking water separators, from which a part of black liquor is transferred into the one or more electrolytic pre-cells, while a thus obtained concentrated suspension of brown cellulose with about 30% w/w dry matter enters into the one or more secondary digesters, with introduction of a white liquor solution from the second cathode compartment of the one or more electrolytic cells; 
 C) transferring a quasi-continuous combined output from all of the secondary digesters with the cooked brown cellulose suspension into a first mixing vessel and a first mill, where separation of milled brown cellulose is performed in a first water separator which concentrates the brown cellulose suspension with 5-15% w/w dry matter up to a content of 27-33% dry matter, and accompanied with separation of the black liquor which is transferred to the one or more electrolytic pre-cells; 
 D) preparing a bleaching suspension in a bleaching reactor, in which, besides the suspension from the step C: 
 regenerated fresh white liquor from the second cathode compartment of the electrolytic cells; 
 mixture of oxygen <02> and chlorine <C12> enveloped in the second anode compartment of the one or more electrolytic cells; and 
 hydrogen peroxide <H 2 O 2 > or sodium peroxide <Na 2 O 2 >, are introduced, yielding the bleaching suspension with a content of 5-15% w/w, 
 E) performing a quasi-continuous bleaching process through two or more parallel bleaching processing lines each including one or more primary reactors, one or more bleaching mills, one or more bleaching water separators, and one or more secondary reactors during 3-6 hours, at temperature from 70-100° C., where feedstock from the step D is prepared and sequentially pumped into said two or more parallel bleaching processing lines, where a primary bleaching process is performed in the one or more primary reactors and where, at the output from the one or more primary reactors, a suspension of bleached cellulose with 8-12% w/w dry matter is obtained, which is subjected to milling in the corresponding one or more bleaching mills and separation in the one or more bleaching water separators from which, a part of waste-solution is transferred into the one or more electrolytic pre-cells, while a concentrated bleached cellulose suspension with about 30% w/w dry matter is entering into the one or more secondary reactors with introduction of: 
 the white liquor solution from the second cathode compartment of the one or more electrolytic cells, 
 the mixture of oxygen <O 2 > and chlorine <Cl 2 > enveloped in the second anode compartment of the one or more electrolytic cells; and 
 the hydrogen peroxide <H 2 O 2 > or the sodium peroxide <Na 2 O 2 >, yielding the cellulose suspension for bleaching with 8-12% w/w; and 
 F) transferring a quasi-continuous combined output from all of the one or more secondary bleaching reactors from the step E into a second mixing vessel and a second water separator, whose waste water is returned back to the one or more electrolytic pre-cells, while a drained cellulose comes out in a form of a pure white cellulose pulp, at a concentration from 48-55% W/W dry matter, with maximally 5% w/w lignin, calculated on the dry matter. 
 
     
     
       2. The continuous process according to  claim 1 , wherein the white cellulose pulp from the step F is further dried in a drier, yielding a dry cellulose powder. 
     
     
       3. The continuous process according to  claim 1 , wherein:
 each of the first and second cathode electrodes is made of carbon steel or rust-free steel, and each of the first and second anode electrodes is made of graphite or magnetite; 
 between the first anode and cathode electrodes in the one or more electrolytic pre-cells and the second anode and cathode electrodes in the one or more electrolytic cells, a voltage of direct current from 1.5-20 V is established, at a current density from 1-10 A/dm 2 , at a temperature of the cell from 80-95° C.; 
 the electrolysis in one or more electrolytic pre-cells is carried out by introducing the black liquor from the step B, the black liquor from the step C, a part of waste solution from the step E, and wastewater from the step F, wherein evolution of the lignin and the non-cellulosic side-products takes place, which are mechanically removed from a top of the electrolyte solution; 
 the electrolyte solution from the one or more electrolytic pre-cells is pumped into the second anode compartment of the one or more electrolytic cells for further processing wherein: 
 (i) at the cathode associated with the second cathode electrode, the white liquor of the following composition is regenerated: 
 (a) 0.50-2.00% w/w NaOH; 
 (b) NaCl; 
 which is distributed into: the preliminary digester in the step A; the one or more secondary digesters in the step B; the bleaching reactor in the step D; and the one or more secondary bleaching reactors in the step E; and generated hydrogen is transferred into a storage tank for hydrogen; 
 (ii) at the anode associated with the second anode electrode, gaseous oxygen <O 2 > and chlorine <Cl 2 > are generated and subsequently introduced into the bleaching reactor in the step D, and into the one or more secondary reactors in the step E; and wherein the rest of the lignin and the non-cellulosic side-products are isolated and mechanically separated from the top of the electrolyte solution. 
 
     
     
       4. The continuous process according to  claim 1 , wherein a concentration of the sodium chloride <NaCl> in the white liquor is 0.50-25.0% w/w. 
     
     
       5. The continuous process according to  claim 4 , wherein the concentration of the sodium chloride <NaCl> in the white liquor is 0.50-1.5% w/w. 
     
     
       6. The continuous process according to  claim 1 , where a starting, form of the grass-like feedstock is in the form of longitudinal pieces, whose fraction by length is minimally 90% between 0.2-2.0 cm, and comprises dried leaves and stems of grass plant species selected from the group consisting of: sorghum/ Sorghum  species, Linne/; maize/ Zea mays , Linne/;  miscanthus/Miscanthus  x  giganteus , Andersson/; sugar beet/Saccharum  officinarum , Linne/; wheat/ Triticuin vulgare , Linne/; hemp/ Cannabis sativa , Linne/; barley/ Horedum vulgare , Linne/; oat/ Avena sativa , Linne/; common flax/ Linum usitatissimum , Linne/; proso millet/ Panicum millaceum , Linne/and other species from the genus  Panicum; triticale/x Triticosecale , Wittm. ex A. Camus/; buckwheat/ Fagopyrum esvulentum , Moench/; rice/ Otyza  sativa, Linne/; esparto grass/ Stipa tenacissima , Linne and  Lygeum spartum , Linne/; reed/ Phragmites australis , Adanson/and other species from the genus  Phragmites ; bagasse from sugarcane processing; jute/ Corchorus olitorius , Linne/; bamboo/ Bambusoideae  spp., Linne/; and their mixtures. 
     
     
       7. The continuous process according to  claim 6 , wherein the starting form of the grass-like feedstock comprises sorghum/ Sorghum  species, Linne/and maize/ Zea mays , Linne/. 
     
     
       8. The continuous process according to  claim 1 , wherein:
 (i) the preliminary digester, the one or more primary digesters, the one or more secondary digesters, the bleaching reactor, the one or more primary reactors, the one or more secondary reactors, and the first and second mixing vessels are equipped with mixing elements which enable intensive stirring of suspended material at more than 900 revolutions per minute; and 
 (ii) the preliminary digester, the one or more secondary digesters, the bleaching reactor, and the one or more secondary reactors are equipped with heating jackets which provide their heating to the working temperature. 
 
     
     
       9. The continuous process according to  claim 6 , wherein the preliminary digester, the one or more secondary digesters, the bleaching reactor, and the one or more secondary reactors are equipped with magnetrons for alternative heating via microwaves. 
     
     
       10. The continuous process according to  claim 8 , wherein:
 (i) the preliminary digester, the one or more primary digesters, and the one or more secondary digesters, 
 (ii) the bleaching reactor, the one or more primary reactors, and the one or more secondary reactors, and 
 (iii) the first and second mixing vessels 
 are additionally equipped with a vibrator to facilitate the mixing, and which generates 10,000-14,000 oscillations per minute. 
 
     
     
       11. The continuous process according to  claim 1 , wherein the quasi-continuous bleaching process is performed at temperature from 70-80° C. 
     
     
       12. The continuous process according to  claim 3 , wherein the electrolytic process for continuous electrolytic separation of the lignin and the non-cellulosic side-products and simultaneous regeneration of the white liquor is carried out at the voltage of direct current from 3-6 V and the current density from 4-6 A/dm 2 . 
     
     
       13. The continuous process according to  claim 1 , wherein the porous membrane in the one or more electrolytic cells comprises at least a material selected from the group consisting of: asbestos, mineral wool, hydrated Portland cement, product of kaolin and sodium silicate, aluminium oxide <Al 2 O 3 >, titanium dioxide <TiO 2 >, zirconium dioxide <ZrO 2 >, polyethylene <PE>, polysulfone <PSU>, polyvinyl pyrrolidone <PVP>, polyvinyl chloride <PVC>, polytetrafluoro ethylene <PTFE>, polyvinylidene fluoride <PVDF>, sulfonated polytetrafluoroethylene <SPTFE>, and composite materials obtained from the combinations of these materials. 
     
     
       14. The continuous process according to  claim 13 , wherein the porous membrane in the one or more electrolytic cells comprises the material selected from:
 zirconium dioxide <ZrO 2 >; from 80-90% w/w, preferably 85% w/w; and, 
 (ii) polysulfone <PSU>; from 10-20% w/w, preferably 15% w/w. 
 
     
     
       15. A process for production of cellulose pulp from a grass-like feedstock that comprises an electrolytic process which is carried out in an electrolytic cell system which includes:
 one or more electrolytic pre-cells, made from a first material inert to the chemistry in which first cathode and anode electrodes are immersed, with no membrane between respective first cathode and anode compartments; 
 one or more electrolytic cells, made from a second material inert to the chemistry in which second cathode and anode electrodes are immersed, where a second cathode compartment is completely separated from a second anode compartment with a porous membrane, which enables electrical contact of an anode and a cathode respectively associated with the second anode and cathode electrodes via ion-exchange, but prevents passing of suspended organic molecules in electrolyte; 
 where the electrolyte is, after processing in the one or more electrolytic pre-cells, transferred into the second anode compartment of the one or more electrolytic cells, and a composition of the electrolyte is modified by addition of a fresh sodium chloride (NaCl) solution into the one or more electrolytic pre-cells; 
 the process comprising: 
 performing quasi-continuous cooking and separation through two or more parallel cooking and separation processing lines each including one or more primary digesters, one or more cooking mills, one or more cooking water separators, and one or more secondary digesters during 3-6 hours, wherein: 
 suspension of the grass-like feedstock in the white liquor during cooking in all of the one or more primary digesters and the one or more secondary digesters is maintained at temperature from 95-100° C.; 
 for each of the two or more parallel cooking and separation processing lines, a brown cellulose suspension at a level 8-12% w/w dry matter at output from the one or more primary digesters is subjected to milling in the corresponding one or more cooking mills and separation in the one or more cooking water separators, from which a part of black liquor is transferred into the one or more electrolytic pre-cells, while a thus obtained concentrated suspension of brown cellulose with about 30% w/w dry matter enters into the one or more secondary digesters, with introduction of a white liquor solution from the second cathode compartment of the one or more electrolytic cells; 
 performing a quasi-continuous bleaching process through two or more parallel bleaching processing lines each including one or more primary reactors, one or more bleaching mills, one or more bleaching water separators, and one or more secondary reactors during 3-6 h, at temperature from 70-100° C., wherein at an output from the one or more primary reactors, a suspension of bleached cellulose with 8-12% w/w dry matter is obtained, which is subjected to milling in the corresponding one or more bleaching mills and separation in the one or more bleaching water separators from which, a part of waste-solution is transferred into the one or more electrolytic pre-cells, while a concentrated bleached cellulose suspension with about 30% w/w dry matter enters the one or more secondary reactors with introduction of: 
 the white liquor solution from the second cathode compartment of the one or more electrolytic cells, 
 the mixture of oxygen <O 2 > and chlorine <Cl 2 > enveloped in the second anode compartment of the one or more electrolytic cells; and 
 yielding the cellulose suspension for bleaching with 8-12% w/w; and 
 transferring a quasi-continuous combined output from all of the one or more secondary bleaching reactors into a second mixing vessel and a second water separator, whose waste water is returned back to the one or more electrolytic pre-cells, while a drained cellulose comes out in a form of a pure white cellulose pulp, at a concentration from 48-55% w/w dry matter, with maximally 5% w/w lignin, calculated on the dry matter. 
 
     
     
       16. The process according to  claim 15 , further comprising, before performing the quasi-continuous cooking and separation, preparing a suspension of comminuted and dedusted grass-like feedstock by addition of a white liquor from the second cathode compartment of the one or more electrolytic cells into a preliminary digester where the grass-like feedstock is subjected to cooking at 80-100° C. in the white liquor solution including the following composition:
 (i) 0.50-2.00 w/w NaOH; 
 (ii) NaCl; 
 by keeping the concentration of the grass-like feedstock at the level of 5-15% w/w. 
 
     
     
       17. The process according to  claim 16 , further comprising, before performing the quasi-continuous bleaching process, transferring a quasi-continuous combined output from all of the one or more secondary digesters with the cooked brown cellulose suspension into a first mixing vessel and a first mill, wherein separation of milled brown cellulose is performed in a first water separator which concentrates the brown cellulose suspension with 5-15% w/w dry matter up to a content of 27-33% dry matter, and accompanied with separation of black liquor which is transferred to the pre-cell of the electrolytic cells; and
 preparing bleaching suspension in a bleaching reactor, in which, besides the suspension: 
 regenerated fresh white liquor from the second cathode compartment of the one or more electrolytic cells; 
 a mixture of oxygen <O 2 > and chlorine <Cl 2 > enveloped in the second anode compartment of the one or more electrolytic cells; and 
 hydrogen peroxide <H 2 O 2 > or sodium peroxide <Na 2 O 2 >, 
 are introduced, yielding the bleaching suspension with a content of 5-15% w/w. 
 
     
     
       18. The process according to  claim 15 , wherein:
 each of the first and second cathode electrodes is made of carbon steel or rust-free steel, and each of the first and second anode electrodes is made of graphite or magnetite; 
 between the first anode and cathode electrodes in the in the one or more electrolytic pre-cells and the second anode and cathode electrodes in the one or more electrolytic cells, a voltage of direct current from 1.5-20 V is established, at a current density from 1-10 A/dm 2 , at a temperature of the cell from 80-95° C.; 
 the electrolysis in the one or more electrolytic pre-cells is carried out by introducing the black liquor and the wastewater into the one or more electrolytic pre-cells where evolution of lignin and non-cellulosic side-products takes place, which are mechanically removed from a top of the electrolyte solution; 
 the electrolyte solution from the one or more pre-cells is pumped into the second anode compartment of the one or more electrolytic cells for further processing wherein:
 (i) at the cathode associated with the second cathode electrode, the white liquor of the following composition is regenerated: 
 (a) 0.50-2.00% w/w NaOH; 
 (b) NaCl; 
 
 which is distributed into: the preliminary digester; the one or more secondary digesters; the bleaching reactor; and the one or more secondary bleaching reactors; and generated hydrogen is transferred into a storage tank for hydrogen;
 (ii) at the anode associated with the second anode electrode, gaseous oxygen <O 2 > and chlorine <Cl 2 > are generated and subsequently introduced into the bleaching reactor and the one or more secondary reactors; wherein the rest of the lignin and non-cellulosic side-products are isolated and mechanically separated from the top of the electrolyte solution. 
 
 
     
     
       19. The process according to  claim 15 , wherein a concentration of sodium the chloride <NaCl> in the white liquor is 0.50-25.0% w/w. 
     
     
       20. The process according to  claim 15 , where a starting form of the grass-like feedstock is in a form of longitudinal pieces, whose fraction by length is minimally 90% between 0.2-2.0 cm, and comprises dried leaves and stems of plant species selected from the group consisting of sorghum/ Sorghum  species, Linne/; maize/ Zea mays , Linne/;  miscanthus/Miscanthus  x  giganteus , Andersson/; sugar beet/ Saccharum officinarum , Linne/; wheat/ Triticum vulgare , Linne/; hemp/ Cannabis sativa , Linne/; barley/ Horedum vulgare , Linne/; oat/ Avena sativa , Linne/; common flax/ Linum usitatissimum , Linne/; proso millet/ Panicum miliaceum , Linne/and other species from the genus  Panicum ; triticale/x  Triticosecale , Wittm. ex A. Camus/; buckwheat/ Fagopyrum esculentum , Moench/; rice/ Oryza sativa , Linne/; esparto grass/ Stipa tenacissima , Linne and  Lygeum spartum , Linne/; reed / Phragmites australis , Adanson/and other species from the genus  Phragmites ; bagasse from sugarcane processing; jute/ Corchorus olitorius , Linne/; bamboo/ Bambusoideae  spp., Linne/; and their mixtures.